Overview
The disclosed apparatus and method relates generally to the field of automated position sensing and automatic inspection control of printing and manufacturing processes.
In manufacturing control systems comprising image capture optics and electronics, positioning of the image capturing mechanism is accomplished via the use of stepper motors or motors with encoder feedback which move the image capturing mechanism in an absolute X and/or Y position over the article of manufacture being inspected. A typical scenario is illustrated in FIG. 1, in which web material (typically paper, but often other items of linear manufacture) are manufactured with perforated punches along either side of the material and rolled in the Y direction beneath a stationary inspection station which includes an optical image capturing device. Mechanical encoders connected to the perforation rollers permit determination of the Y position of the material as it moves past the image inspection station. In the illustrated example of FIG. 1, the image capturing device is movable in the X direction laterally across the surface of the web material.
Once properly positioned over the article of manufacture, images may be captured of the article as it progresses along the manufacturing or assembly line process and these images may then be processed to detect and correct manufacturing flaws or make adjustments in the pre-inspection manufacturing controls to optimize product characteristics.
Critical Issue
A critical aspect of this inspection and manufacturing feedback control system is determining the absolute position of the image capturing mechanism with respect to the article of manufacturing being inspected. This position sensing function is complicated by the following issues:
For example, the typical traverse for lateral positioning of the camera has a 0.001 inch encoder resolution but a repeatability of the positioning mechanism of .+-.40 mils. Thus, the accuracy of the positioning mechanism is limited by the repeatability and is thus .+-.40 mils for a total variation of 80 mils or 0.08 inch.
When both mechanical wear and thermal variations are combined, it is seen that the overall web production system has a moving measurement dynamic, and as such any effective and accurate calibration procedure performed to support web inspection/control functions (i.e. initial register, repeat length) must include the calibration procedure as an integral part of the web inspection/control function. The Zoom Calibration method described herein is designed specifically to be incorporated within the context of any practical web inspection/control protocol, thus enabling rapid and accurate distance measurement calibration before the web inspection/control function is performed.
Prior art approaches to distance calibration do not take this approach, as all conventional calibration procedures are manual and performed with the use of an operator. As a result, none of the prior art is capable of compensating for both thermal and mechanical variations in web manufacturing equipment. While it is possible for an operator to perform manual calibration procedures on an accelerated schedule (as is done in some manufacturing lines), these extra calibration procedures only serve to reduce the overall efficiency of the manufacturing process and increase the overall cost of web production.
While the requirement for frequent calibrations of any mark detection device is dictated by the realities of the web production system, it is clear from the cited patents that the prior art teaches away from this method for economic reasons. As a result, conventional web manufacturing processes rely almost exclusively on initial operator calibration of register mark detection devices.
So, even if highly accurate encoder and positioning mechanisms are employed with conventional calibration technologies, the results are economically impractical due to the overall slow positioning speed of the image capture device (camera). This slow positioning speed also makes multitasking of the image capturing device impractical, since the key to multitasking is amortizing the image capturing/processing cost over a wide range of web inspection/control functions. If the time required to perform a given web inspection/control function is too large, then the total time to cycle through all web inspection/control functions will be prohibitive.
Thus, the speed/positioning tradeoff inherent in conventional web inspection designs explains why these systems have been exclusively single-purpose, as well as explaining why web press manufacturers have been reluctant to include more than a very minimal set of web inspection/control functions in a particular press configuration. The key capability of rapid distance calibration has been lacking in these previously disclosed systems. The economics of including a multiplicity of web inspection/control functions on a single press quickly increases the press cost to be economically prohibitive using these previous approaches. As a result, much of the manual operative nature of these web presses has remained unchanged for decades.
Note that if the speed of the traverse and/or zoom is increased, then the positioning accuracy is sacrificed, and can reach a .+-.20% error at high positioning/zoom rates, eliminating any benefit to be had from the use of more expensive encoders and positioning mechanisms. Thus, an infusion of money into the quality of the encoders/positioners in a given inspection system will not necessarily produce more accurate results as would be posited by conventional wisdom.
Note, however, that the use of more accurate encoders and positioning mechanisms in conjunction with the Zoom Calibration method described herein can, when combined with sufficiently accurate image capturing mechanics, provide improved accuracy commensurate with the increase in encoder/positioner accuracy. The key to this improvement is the ability of the Zoom Calibration method to compensate for variations in all the system inaccuracies (encoder, positioner, lens, mechanical wear, thermal expansion, image capture distortions, etc.).
The speed/position tradeoff described above is not unknown in the prior art, and can only be corrected by dynamically calibrating the speed and position of the image capture device during the measurement process as described in the disclosed Zoom Calibration method detailed later in this document.
Some methods to reduce this error have been tried including running the zoom to a stop and removing backlash by always approaching a position from the same direction. Running the zoom to a stop restricts the use only to the ends of the zoom. Removing backlash has not proved successful in commercial practice. Reasons for this are well known in the art, but include limited accuracy with analog positioning mechanisms, gear backlash and wear, as well as the ever present variations caused by temperature variations within the positioning system. Additional sources of error can include limited encoder resolution as well as stepper motor wear and differentials in analog encoder output due to potentiometer contamination.